Rev. means Revision, I believe we're on Revision D now. Since they haven't undergone any major body/construction changes(as with the MacBooks from Plastic, unibody, polycarbonite, glossy, matte, etc), the only real way to differentiate is with the version that is it. Why it's not 1-5, I don't know.
We're on Rev E.
Rev A - Original MacBook Air - January 2008 (1.6GHz and 1.8GHz Merom Core 2 Duo processors, PATA HDD and SSD options, Intel GMA 3100 graphics)
Rev B - November 2008 - (1.6GHz and 1.86GHz Penryn Core 2 Duo processors, SATA HDD and SSD options, NVIDIA 9400m graphics)
Rev C - June 2009 - (1.86GHz and 2.13GHz Penryn Core 2 Duo processors, slightly higher capacity battery, faster Toshiba SSDs entered in later in the cycle, otherwise similar to the Rev B).
Rev D - October 2010 (11" option - 1.4GHz and 1.6GHz Penryn Core 2 Duo, 13" option -1.86GHz and 2.13GHz Penryn Core 2 Duo, all models with SSD storage and NVIDIA 320m graphics, 2nd USB port, new case designs)
Rev E - July 2011 (11" option 1.6GHz Sandy Bridge Core i5 and 1.8GHz Sandy Bridge Core i7, 13" option 1.7GHz Sandy Bridge Core i5 and 1.8GHz Sandy Bridge Core i7, Intel HD 3000 graphics, Thunderbolt Port, otherwise similar to the Rev E).
People sometimes call Rev A-C the "first generation" and the Rev D and E the "second generation," as well, since the second generation added the second USB port, changed the case design and heat management system, and made SSDs standard.